In the manufacture of alkaline pulp for the production of paper, wood is delignified with caustic soda, and a profitable operation requires that the soda be eventually recovered for re-use. In the recovery procedure, the process liquor is concentrated and burned to remove organic materials.
In general, alkaline pulp is made by subjecting wood chips to caustic soda together with the application of heat and pressure by injecting steam into a closed system. The non-cellulose constituents of the wood are hydrolyzed and rendered soluble, enabling the cellulose fibers to be separated.
Following the cooking procedure, the liquor is recovered and is constructed by evaporating prior to burning. Until recent times, the residual organic residues in the liquor were ignored and simply burned to reclaim the inorganic soda. One of the by-products in the processing liquor is referred to as tall oil, derived from the Swedish term tallol. Tall oil is a liquor composed mainly of resin acids and fatty acids in varying proportions, according to the character of the wood and the cooking methods employed. More recently, however, tall oil soap has become an important by-product that is recovered from the concentrated liquor by floatation. The tall oil soap is then acidulated and converted to the acid form and allowed to separated. A large amount of tall oil, upon acidulation and separation, remains in a sludge layer, and if not somehow separated, will be lost as waste or only burned for its heat value and not recovered for its much higher tall oil value.